A primer on system components and how they operate.
By Mark Quasius, F333630
Hydraulic brakes perform well on relatively lightweight motorhomes, but heavy vehicles need more braking capacity. That’s why diesel-powered Type A motorhomes usually are equipped with air brakes. Read on to gain a better understanding of air brake systems. Next month, we’ll focus on how to test and troubleshoot them.
Drum Versus Disc
Drum brakes have been the standard for many years. They operate by expanding a pair of shoes inside a spinning brake drum. Braking force is applied as the shoes contact the inner surface of the drum.
Disc brakes first began appearing on passenger cars in the late 1960s, but it took another 40 years before heavy-duty disc air brakes began to appear on RV chassis. Disc brakes consist of a spinning rotor that is clamped by a caliper — very much like a multispeed bicycle caliper grabbing the wheel rim.
Drum brakes have limitations. When cold, they are not as effective as warm brakes. And if cold drum brakes get wet, stopping power is reduced until the drum heats up and dries out.
Drum brakes function best at an operating temperature of about 475 degrees. If the brakes are used repetitively, such as when descending a grade or navigating a series of switchbacks, the heat builds up within the drum, raising the temperature to excessive levels. Drum brake temperatures above 800 degrees can lead to brake fade, reduced stopping distance, and warped brake drums.
Disc brakes have better stopping power. The disc pads constantly ride on the rotor so they maintain an effective braking temperature. Disc brakes also aren’t affected by water as much as drum brakes. And, because the disc rotor is in the open air, repetitive use doesn’t cause it to overheat like an enclosed drum. Many rotors are vented to allow for even faster cooling. Disc brake rotors can warp, but only when subjected to an extreme amount of heat. Some rotors, such as Spartan Chassis’ cryogenically treated rotors, can handle even extreme temperatures.
Powering The Brakes
Hydraulic brake systems move hydraulic brake fluid from a master cylinder to each wheel. The effort needed to provide adequate stopping force is substantial, so vacuum-powered boosters or similar devices help to minimize the effort required at the brake pedal. Brake fluid is hygroscopic, which means it attracts and holds water. When the brakes heat up, water can turn into steam and result in a spongy pedal and brake fade. Therefore, it’s recommended that the hydraulic brake fluid be replaced every two to three years.
Air brakes, which operate with compressed air, have numerous advantages over hydraulic brakes. An engine-driven compressor provides the air, which is constantly being replaced and never needs to be changed. Also, pedal effort is minimal, because it simply involves depressing a treadle valve that sends air to the brake relay valves. The air does need to be kept clean and dry, but the air dryer handles that.
Auxiliary Braking Systems
Most diesel-powered motorhomes are equipped with an auxiliary braking system, such as an engine compression release brake (for example, the Jacobs Vehicle Systems Jake brake) or a transmission retarder. Auxiliary systems are not intended to be the primary brakes, but they do help to slow a motorhome on long grades and thereby prevent riding and overheating those brakes.
Because brakes operate best when warm, the Jake brake generally should be used sparingly. If the engine brake is on every time the motorhome comes to a normal stop, not much pressure will be applied to the service brakes, which can lead to glazing and reduced performance. The typical life expectancy of motorhome service brakes is 350,000 miles, so don’t worry about wearing them out.
The air that powers the brakes is delivered by an engine-driven air compressor.
Ambient air is compressed and sent to storage tanks to be used by the brake system. The air pressure is regulated by a governor that controls the air’s cutout pressure. When the pressure builds to the cutout pressure — designed to be a minimum of 115 psi and a maximum of 135 psi — the governor sends an air signal to the compressor valves so that the compressor runs unloaded until the pressure drops 25 psi and begins pumping again. Most chassis manufacturers set the governor to cut out at 130 psi and cut in at 105 psi.
The brake system needs clean and dry air. Moisture in the system can freeze in cold weather, preventing the brakes from working. Oil and other contaminants can damage the system’s rubber seals and diaphragms. An air dryer is not required by law, but most air-brake-equipped vehicles built today have one.
The dryers use a replaceable desiccant filter cartridge to remove moisture and contaminants that are produced while compressing air. Most of these contaminants rest in the base of the unit. A heated ejector valve is connected to the air compressor’s governor. When the air pressure reaches the cutout point, the governor unloads the compressor valves and sends a pneumatic signal to the dryer. The dryer then opens the purge valve, which is why an air spurt can be heard as moisture or debris is ejected. Anything not ejected eventually will be trapped in the filter. The filter should be replaced according to the motorhome’s service schedule or if excessive moisture appears when one is manually operating the air tank drain valves.
Air Tanks And Lines
The typical storage tank system consists of three air tanks. Once air leaves the compressor and passes through the dryer, it arrives at the wet tank. If moisture or condensation exists, it should condense and settle in this tank, which also serves as a buffer to remove any pulsing in the air as it passes down the line.
From the wet tank, air then goes to the primary and secondary tanks. The primary tank operates the rear brakes; the secondary tank operates the front brakes. In cases where there may appear to be only two air tanks, the primary tank is divided into two separate compartments with primary and wet chambers. Both the primary and secondary tanks have pressure gauges located in the instrument panel, as well as a low-pressure alarm that will alert the driver any time the pressure in a system drops below 60 psi.
Safety valves on each tank are preset to open at 150 psi. Brake lines typically follow a standard color coding, with green lines for the primary brakes and red lines for the secondary brakes. Drain valves are on each tank. Manual drain valves usually are operated by pulling lanyard cables in one of the wheel wells, but some motorhomes may have remote petcocks in a basement compartment, so be sure to check the owners manual for the location.On my Spartan chassis, each lanyard is color-coded so that I know which tank I am draining.
A motorhome may be equipped with automatic drain valves as well. These are required by law if the coach is not equipped with a wet tank. But, chances are the motorhome will have them anyway, even if there is a wet tank. The valves momentarily expel moisture when the internal pressure differential is between 15 and 18 psi. The valves may have a heater to prevent freezing in cold weather.
Even if the motorhome is equipped with automatic tank drains, it is still important to manually drain the tanks as part of a regular inspection process. If excessive oil, moisture, or desiccant powder comes out, it’s time to service the air dryer.
On a motorhome with drum brakes, the air brakes are equipped with brake shoes and drums similar to a hydraulic brake system, but the mechanism to operate them is vastly different. Instead of hydraulic wheel cylinders that expand the shoes, air brakes employ a shaft with cams on the end. The brake shoes expand when the shaft is twisted, and the cams act upon the rollers on the end of the brake shoes. The shaft extends inboard and away from the wheel and is connected to a brake diaphragm chamber via an arm called a slack adjuster.
Slack adjusters are adjustable so that the brake diaphragm need only move a certain amount to apply the brakes. Today’s motorhomes have automatic slack adjusters, so under normal operating conditions they should remain properly adjusted. If the coach sits for long periods, the cams may rust, and the brakes might stick and not apply smoothly. In that case, the cams and rollers should be serviced and cleaned.
Brake diaphragms are fairly large in diameter, maybe 6 to 10 inches. A pushrod exits the center of the brake chamber and connects to the slack adjuster, which operates the brake shaft. If the brakes are worn or the slack adjuster is not properly adjusted, the shaft will expose a red band that indicates excessive extension of the chamber’s shaft. When that happens, it’s time for brake service.
The front and tag axles have brake diaphragms, while drive axles have spring brakes. A spring brake, which functions as a parking brake, actuates with spring pressure. It ensures that a motorhome won’t roll away should the air pressure drop; it also stops a moving coach in the event of a total failure of the air system.
The spring brake automatically deploys when the pressure drops below 60 psi. To release the brake, first ensure there is air pressure, and then push the yellow park brake knob. Pressure will be applied to the front chamber of the dual-chamber spring brake to release it. So, in effect, a spring brake uses air pressure to release the brake, rather than apply it.
As noted, spring brakes are found only on drive axles. So, when camping, the drive axle must be kept on the ground in order for the spring brake to serve as a parking brake. Spring brakes are never used on the front axle; if the motorhome were in transit and the air pressure dropped, locked-up front brakes would make steering impossible. During normal driving, air pressure is applied to the drive axle to stop the vehicle. That happens through the anti-compounding valve, which we’ll discuss a bit later.
Disc brakes don’t have slack adjusters, brake shoes, or rotary cams. They use brake diaphragms to apply pressure to the calipers, some of which are totally sealed. They are self-adjusting but manage that through a different and less complex method.
Valves And Controls
A one-way check valve installed on the primary tank allows air to enter and prevents air from flowing in the opposite direction. The secondary tank is equipped with a pressure control check valve (PCCV) if the system uses a regenerative-style dryer, such as one made by Wabco. The PCCV valve lets air flow into the secondary tank at all times but allows air to flow back to the wet tank only if the pressure is above 95 psi. This maintains a minimum pressure in the secondary tank while allowing the air dryer to use 10 psi of clean air from the secondary tank during the dryer’s regeneration purge.
The dual foot control or treadle valve applies air to both the primary and secondary braking systems in response to the driver’s foot pressure. The primary valve is applied mechanically via the pedal while the secondary valve is applied via air from the primary side. If the primary side has failed and has no air pressure, the secondary side is mechanically applied. Spartan sets the foot control valve to apply 3 to 5 psi of air to the primary side before applying the secondary side. This applies the rear brakes slightly before the front brakes to minimize any front end nosedive while braking.
On the secondary system, quick-release valves discharge air pressure from the front axle brake chambers to disengage the brakes. On some brake systems, you can hear the valves make a honking sound when the brake pedal is released. Service relay valves are used on the primary system to improve brake balance on the longer runs from the foot pedal to the rear brakes. The relay valves are similar to the quick-release valves but are connected directly to a reservoir for faster response and better airflow control.
The dash control valve, commonly called the push-pull valve, is used to deploy the parking brake. Pushing in the yellow knob applies air pressure to the parking brake to release it. Pulling out the knob exhausts air pressure from the parking brake, allowing spring force to apply it. If the system air pressure drops below 20 psi, the yellow knob automatically pops out. (Keep in mind that the spring brakes will have already been applied once the air pressure drops to 60 psi.)
If a massive failure of the air supply system occurs, the rear brake will not totally lock up (which could cause loss of control), but the brakes will bring the vehicle to a fairly abrupt stop. A two-way check valve receives air from both the primary and secondary systems; the valve allows air from the dominant system to flow to the dash control valve so that the parking brake can be released if one system experiences a failure.
Spring brakes use spring tension to apply the parking brake; an air chamber releases the parking brake. A second chamber for the service side of the system applies the service brakes when driving.
The spring in a 30-inch spring chamber produces about 1,800 pounds of force when the motorhome is parked. A spring brake connected to a 6-inch slack adjuster arm produces 10,800 inch-pounds of torque. At the same time, the driver steps on the pedal and delivers 50 psi to the service side of the spring brake. So, 50 psi multiplied by 30 square inches equals 1,500 pounds of force. And, 1,500 pounds multiplied by the 6-inch slack arm equals 9,000 inch-pounds of additional force. This is compounding.
Compounding can lead to premature cracking or breaking of brake drums, bent chamber push rods, mounting studs torn out of the spring brakes, broken slack adjusters, or torn spring brake mounting brackets on the axle.
Although not mandated by law, an anti-compounding system can prevent such damage. This is accomplished with an anti-compounding valve, which is a quick-release valve with a double-check valve built into it. It applies air pressure to the parking brake chamber at the same time that air is applied to the service brake chamber. As a result, the appropriate amount of park brake pressure is released in relation to the service brake pressure, preventing excessive torque from damaging the spring brakes.
An inversion valve has been mandated by law since 1975. Under normal conditions, this valve does not need to operate. It serves only as an emergency valve in case the primary air system fails when the vehicle is being driven. If the air pressure in the primary system drops to zero, the parking brakes will be held in the released position with air from the secondary tank. The inversion valve receives the signal from the foot pedal and equally releases pressure on the spring brake chamber to apply the service brakes mechanically with power from the spring portion. When the brake pedal is released, air is restored to the spring brake chamber to release the brakes.
ABS And Automatic Traction Control
Antilock braking systems (ABS) release the brakes when wheels lock up during a hard stop. On every wheel position are a tooth wheel and electronic sensor that recognize when the wheels have stopped turning. The electronic brain then communicates with a valve that removes air pressure from a wheel to allow it to resume turning. The valves typically can perform this function up to five times per second.
If you’re in a panic stop or begin to slide in slippery conditions, keep your foot on the brake pedal and allow the ABS to modulate the brake pressure. Backing off the pedal will be counterproductive and will increase the stopping distance.
Automatic traction control (ATC) is another safety feature commonly found on higher-horsepower coaches. It employs the same wheel position sensors, but it monitors wheel spin rather than wheel lockup, and it communicates with an electronic control module. If the throttle is applied too hard in wet conditions, the ATC detects wheel spin and directs the ABS modulator valve to apply brake pressure to that wheel to regain traction. The ATC also can interface with the engine and will disconnect the cruise control if it’s in use.
Proper Startup Procedure
ABS is rarely used. It’s like an insurance policy — invaluable if the need arises. However, sitting idle isn’t the best for any vehicle component. The ABS ejector ports can freeze up from lack of use. Spartan Chassis recommends using the following procedure to exercise the ABS every time the motorhome is started:
- Apply the foot brake and hold it with medium pressure.
- Turn the ignition key on, but do not start the engine; continue to hold the brake pedal down.
- Listen for four or six pops (one for each wheel) as each ABS modulator cycles through and expels air through the modulator exhaust port.
- Start the engine and release the park brake.
- Apply the foot brake with full pedal pressure, then release it. This operates the self-adjusters to keep the brakes in proper adjustment.
Now that we’ve covered how an air brake system operates, the next issue of FMC will describe important pretrip inspections, as well as tests and troubleshooting procedures that motorhome owners can perform.
Hydraulic brakes perform well on relatively lightweight motorhomes, but heavy vehicles need more braking capacity. That’s why diesel-powered Type A motorhomes usually are equipped with air brakes. Read on to gain a better understanding of air brake systems.